Churchyards and private farmlands throughout the German state of Bavaria are perforated from below by "more than 700 curious tunnel networks" whose "purpose remains a mystery."

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As Der Spiegel reports, "The tunnel entrances are sometimes located in the kitchens of old farmhouses, near churches and cemeteries or in the middle of a forest. The atmosphere inside is dark and oppressive, much as it would be inside an animal den."

Although the subterranean networks are considered an "extremely unusual ancient phenomenon," other "small underground labyrinths have been found across Europe, from Hungary to Spain, but no one knows why they were built."

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Small might actually understate the case: indeed, "the tunnels are often only 20 to 50 meters long. The larger passageways are big enough so that people can walk through them in a hunched position, but some tunnels are so small that explorers have to get down on all fours. The tiniest passageways, known as "Schlupfe" ("slips"), are barely 40 centimeters (16 inches) in diameter."

I'm particularly fascinated by examples of these tunnels being found on what is now private property. For instance, a family named the Greithanners, "from the town of Glonn near Munich, are the owners of a strange subterranean landmark. A labyrinth of vaults known as an Erdstall runs underneath their property. It is at least 25 meters (82 feet) long and likely stems from the Middle Ages." I'm genuinely curious what the legal status of such discoveries might be. If, for instance, you discover someday that your house sits atop hundreds of feet of artificially excavated underground space from the Middle Ages, do your property taxes go up-or down, due to the structural inconvenience of owning land hollowed out from below?

In any case, Der Spiegel goes on to explain how local archaeologists (who, in order to avoid underground suffocation, once "blew air into a tunnel with a 'reversible vacuum cleaner'") have teamed up with engineers to explore these spaces-including a man named Nikolaus Arndt, who earlier in his career helped to build the Great Man-Made River of Libya. For now, the tunnels' original purpose still remains unclear:

The vaults could not have served a practical purpose, as dwellings or to store food, for example, if only because the tunnels are so inconveniently narrow in places. Besides, some fill up with water in the winter. Also, the lack of evidence of feces indicates that they were not used to house livestock.

There is not a single written record of the construction of an Erdstall dating from the medieval period. "The tunnels were completely hushed up," says [Dieter Ahlborn, leader of the Working Group for Erdstall Research].

Archeologists have also been surprised to find that the tunnels are almost completely empty and appear to be swept clean, as if they were abodes for the spirits. One gallery contained an iron plowshare, while heavy millstones were found in three others. Virtually nothing else has turned up in the vaults.

The rest of the occasionally bizarre article-one of the locals, for instance, says that sitting alone inside an Erdstall makes him "feel like a Hopi Indian"-is worth reading, though any hope that these tunnels might someday be found to rival the discovery of Derinkuyu should, alas, be put aside. Read more at Der Spiegel.

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(Thanks to Derek Upham for the tip!)

Photos and diagram by Ben Behnke, courtesy of Der Spiegel. This post originally appeared on BLDGBLOG.